The present invention relates to an improvement in a transferred plasma heating anode and, particularly, to a transferred plasma heating anode suitable for heating a molten steel in a tundish.
FIG. 1 shows a direct current twin-torch plasma heating device used for heating a molten steel in a tundish. Two plasma torches, an anode 3 and a cathode 4, are inserted through a tundish cover 2, and a plasma arc 6 is generated between the torches 3, 4 and a molten steel 5 to heat the molten steel. An electric current 7 flows from the cathode 4 to the anode 3 through the molten steel 5.
One example of an anode plasma torch is shown in FIG. 2. FIG. 2 shows a cross section of the tip end portion of the anode torch. For example, oxygen-free copper is used as a material for the anode 3. The anode torch comprises an outer cylinder nozzle 8 that is made of a stainless steel or copper and that covers the outside and the anode 3 that is made of copper and that is situated inside the torch. The tip end portion of the anode 3 is in a flat disc-like shape. Both the anode 3 and the outer cylinder nozzle 8 each have a cooling structure. The inlet side and outlet side water paths of cooling water of the anode 3 are partitioned with a partition 9; the inlet side and outlet side water paths of cooling water of the outer cylinder nozzle 8 are partitioned with a partition 11 (reference numerals 10, 12 in FIG. 2 indicating the flows of cooling water). There is a gap 13 between the outer cylinder nozzle 8 and the anode 3, and a plasma gas is blown from the gap 13.
One of the problems associated with the direct current anode plasma torch is that its life is short because the anode tip end is damaged. Because the anode becomes a receiver of electrons during plasma heating operation, electrons strike the external surface of the anode tip end, and the thermal load applied to the tip end external surface becomes significant.
Moreover, the thermal load applied to the anode tip end is as large as several tens of megawatts/m2, and the form of heat transfer on the cooling side at the anode tip end is thought to be a heat transfer through forced-convection nucleate boiling. When the heat transfer is through forced-convection nucleate boiling, the heat transfer rate is a magnitude of 105[W/m2K], and is about 10 times as large as that of a forced-convection heat transfer. When the thermal load applied to the external surface of the anode tip end becomes excessive, the temperature of the heat transfer surface on the cooling side rises, and a burnout phenomenon in which the heat transfer form changes from nucleate boiling to film boiling takes place. When the change takes place, the heat transfer rate rapidly lowers on the heat transfer surface, and the heat transfer surface temperature rises. Finally, the temperature of the anode tip end exceeds the melting point, and there is a possibility that the anode tip end is melted and lost.
For the conventional anode cooling water path structure shown in FIG. 2, a thermal load that causes burnout, namely, a burnout critical heat flux is shown in FIG. 31. In the graph shown in FIG. 31, a radius on the tip end cooling side of the anode 3 in which the maximum radius Rcool on the tip end cooling side thereof is 22 mm is taken as abscissa, and a burnout critical heat flux is taken as ordinate. Zenkevich""s formula (Zenkevich et al, J. Nuclear Energy, Part B, 1-2, 137, 1959) is used for estimating the burnout critical heat flux, and the burnout critical heat flux WB0 [W/m2] is expressed by the formula (1):
WBO=L{square root over (("sgr"G/"sgr"))}(2.5+184(ixe2x88x92icool)/L)xc3x9710xe2x88x925xe2x80x83xe2x80x83(1) 
wherein L, "sgr", G, xcexd, i and icool in the formula (1) are physical quantities, L is a heat of vaporization [J/kg], "sgr" is a surface tension [N/m], G is a weight speed [kg/m2s], xcexd is a kinematic viscosity [m2/s], i is an enthalpy [J/kg] and icool is an enthalpy [J/kg] of a main stream. It is seen from the graph in FIG. 31 that the burnout critical heat flux near the center is low. The heat flux is low because the influence of the flow rate of the cooling water flowing in the anode 3 is significant. The cooling water flowing from the upper side of the anode in the central portion strikes the anode tip end to lower the flow speed. As a result, the burnout critical heat flux is also lowered. When the thermal load applied to the external surface of the anode tip end exceeds the burnout critical heat flux, it is estimated that burnout takes place on the cooling side of the anode tip end to raise the heat transfer surface temperature and to melt the anode tip end. The central portion of the anode tip end where the burnout critical heat flux is low therefore tends to be melted and lost.
Moreover, when transferred plasma heating is conducted, heat tends to concentrate on the central portion of the external surface of the anode tip end. Furthermore, when a current concentration site (anode spot) is once formed on the anode surface, current further tends to concentrate on the anode spot. That is, when damage begins to be formed on the external surface of the anode tip end due to melting, formation of the damage is further promoted, and the damage finally reaches the cooling water side to end the life of the anode.
FIG. 3 illustrates the pinch effect associated with plasma. A flow 14 of a gas having temperature sufficiently lower than that of plasma 15 blown from a gap 13 between an outer cylinder nozzle 8 and an anode 3 concentrates the plasma 15 in the central direction (thermal pinch effect). In general, the current density in plasma is described as an increasing function of temperature, and the current density in a plasma central portion 16 is large in comparison with the average. As a result, the current density incident on a central portion 17 of the external surface of the anode tip increases. Accordingly, the degree of damage is large in the central portion 17 on the external surface of the anode tip end in comparison with a peripheral portion 18 of the external surface at the tip end. Moreover, electrons 21 moving toward the anode in the plasma receive a force 22 directing toward the central portion by interaction with a rotating magnetic field 20 produced by a current 19 flowing in the plasma (magnetic pinch effect).
Furthermore, as shown in FIG. 4, the anode tip end is outwardly deformed in a protruded shape by the pressure of the cooling water flowing inside, thermal stress and creep. The protruded deformation forms a projection 23 in the central portion 17 of the external surface of the anode tip end. As a result, an electric field 32 is concentrated on the projection 23. Since electrons 21 moving in the plasma are accelerated in the direction of the electric field 32, the current 19 is concentrated on the projection 23. Accordingly, the electric current is further concentrated on the central portion 17 of the external surface at the anode tip end. That is, the central portion 17 of the external surface at the anode tip end is further likely to be damaged. When the damage is increased in the central portion 17 of the external surface at the anode tip end, a cooling water path 25 of the anode is finally broken, and operation becomes impossible. As explained above, as a result of concentrating an electric current on the central portion 17 of the external surface at the anode tip end, the life of the anode is significantly shortened.
FIGS. 5(a) to 5(d) illustrate the concentration of an electric current on an anode spot. In an initial state (FIG. 5(a)) in which the cleanness of an external surface 26 of the anode tip end is excellent, electrons 21 are approximately vertically incident on the external surface 26. However, as explained above (see FIG. 4), an electric current tends to concentrate on the central portion 17 of the external surface at the anode tip end. When the external surface 26 is heated to a high temperature, the copper is melted and evaporated to form a vapor cloud 27 of a copper vapor near the center of the external surface (FIG. 5(b)).
When electrons strike the vapor cloud 27, the electrons in the evaporated copper atoms 28 are excited and ionized. Electrons 29 ionized from the copper atoms each have a small mass, and show a large mobility, therefore, the electrons are incident on the external surface of the anode tip end. However, since copper ions 30 show a small mobility and stay in the vapor cloud 27, the vapor cloud 27 is positively charged (FIG. 5(c)).
The positive charge potential of the vapor cloud 27 accelerates the electrons 21 in the plasma arc toward the vapor cloud 27 (FIG. 5(d)).
Consequently, when an anode spot 31 is formed, electrons in the plasma arc near the external surface 26 of the anode tip end are acceleratedly centered on the central portion of the external surface at the anode tip end. Damage at the anode tip end is acceleratedly increased by such a mechanism.
The present invention relates to the shape and material of the anode tip end in a plasma heating anode that allows a burnout critical heat flux to be influenced by cooling, and that delays damage to the anode tip end to extend the life of the anode.
In order to solve the above problems, the present inventors provide the present invention, aspects of which are described below.
(1) A transferred plasma heating anode for heating a molten metal in a container by applying an Ar plasma generated by passing a direct current through the molten metal, the transfer mode of plasma heating anode comprising; an anode composed of a conductive metal that has an internal cooling structure, a metal protector having an internal cooling structure that is placed outside the anode with a constant gap between the anode and the protector, and a gas supply means that supplies an Ar-containing gas to the gap, is characterized by the central portion on the external surface of the anode tip end being inwardly recessed.
(2) A transferred plasma heating anode for heating a molten metal in a container by applying an Ar plasma generated by passing a direct current through the molten metal, the transferred plasma heating anode comprising; an anode composed of a conductive metal that has an internal cooling structure, a metal protector having an internal cooling structure that is placed outside the anode with a constant gap between the anode and the protector, and a gas supply means that supplies an Ar-containing gas to the gap, is characterized by the whole of the external surface of the anode tip end being inwardly recessed.
(3) A transferred plasma heating anode for heating a molten metal in a container by applying an Ar plasma generated by passing a direct current through the molten metal, the transfer mode of plasma heating anode comprising; an anode composed of a conductive metal that has an internal cooling structure, a metal protector having an internal cooling structure that is placed outside the anode with a constant gap between the anode and the protector, and a gas supply means that supplies an Ar-containing gas to the gap, is characterized by the cooling surface of the anode tip end having ribs.
(4) A transferred plasma heating anode for heating a molten metal in a container by applying an Ar plasma generated by passing a direct current through the molten metal, the transfer mode of plasma heating anode comprising; an anode composed of a conductive metal that has an internal cooling structure, a metal protector having an internal cooling structure that is placed outside the anode with a constant gap between the anode and the protector, a first gas supply means that supplies an Ar-containing gas to the gap, and a second gas supply means in the interior of the anode, is characterized by the second gas supply means having a function of blowing a gas from the external surface of the anode tip end.
(5) The transferred plasma heating anode according to (1), wherein the central portion and the whole of the external surface of the anode tip end are inwardly recessed.
(6) A transferred plasma heating anode for heating a molten metal in a container by applying an Ar plasma generated by passing a direct current through the molten metal, the transferred plasma heating anode comprising; an anode composed of a conductive metal that has an internal cooling structure, a metal protector having an internal cooling structure that is placed outside the anode with a constant gap between the anode and the protector, and a gas supply means that supplies an Ar-containing gas to the gap, is characterized by the center on the cooling side of the anode tip having a projection.
(7) The transferred plasma heating anode according to (6), wherein the central portion of the external surface of the anode tip end is inwardly recessed.
(8) The transferred plasma heating anode according to (6) or (7), wherein the whole of the external surface of the anode tip end is inwardly recessed.
(9) The transferred plasma heating anode according to any one of (1), (2), (5) and (6) to (8), wherein the cooling side of the anode tip end has ribs.
(10) The transferred plasma heating anode according to any one of (1) to (3), (5) and (6) to (9), wherein the anode has a second gas supply means in the interior of the anode, and the second gas supply means has a function of blowing a gas from the external surface of the anode tip end.
(11) The transferred plasma heating anode according to any one of (1) to (10), wherein the entire and/or central portion of the external surface of the anode tip end is recessed, and the anode has in the interior of the anode one or at least two permanent magnets freely rotatable in the circumferential direction.
(12) The transferred plasma heating anode according to any one of (1) to (11), wherein the material of at least the anode tip end is a copper alloy containing Cr or Zr.